Embodiments of the invention relate to field effect transistors such as MOSFETs (metal oxide semiconductor field effect transistors) and methods for making field effect transistors.
Laterally Diffused MOS (LDMOS) structures are widely used in high voltage transistors. LDMOS transistors can provide wide frequency range, high linearity, good ruggedness performance, and high breakdown voltages. A conventional LDMOS transistor has its contacted source and drain regions near the surface of the semiconductor wafer, and thus, the current flow in the transistor is more or less along the lateral dimension. In an alternative design, LDMOS transistors have drain contacts along the backside of the die. LDMOS transistors with backside drain typically have a structure arranged in a horizontal sequence of source, polysilicon gate, lightly doped drain (LDD), and sinker region. This arrangement tends to result in a large device size. On the drain side of the transistor, the LDD region is often extended laterally to obtain high voltage. Moreover, the sinker region needs to be diffused sufficiently to reach the backside drain. Such deep diffusion tends to consume additional die area due to side diffusion and misalignment.
Thus, there is a need for a LDMOS structure with a small cell pitch and superior transistor performance, which can be formed using a simple manufacturing process.